1. Field of the Invention
The invention relates to a transmission cable constructed by multilayer technique, said cable being located in a cavity with a first surface and a second surface essentially parallel to the first, said transmission cable consisting of a signal cable that is essentially parallel with the first surface of the cavity, and of a ground cable that is placed on said second surface essentially in parallel with the signal cable.
2. Brief Description of Related Developments
Various different cable structures are utilised in the construction of electronic appliances. As the frequencies of operation increase, there are higher requirements set for the cable structures to be used, in order to prevent attenuation caused by said cable structures. At present, in the structures of electronic appliances, there is generally applied the so-called multilayer technique, which is based either on the HTCC technique (High Temperature Cofired Ceramics) or on the LTCC technique (Low Temperature Cofired Ceramics). With both manufacturing methods, the produced structures consist of several green tapes, with a thickness of about 100 xcexcm, which are positioned one on top of the other. Prior to thermal treatment, the material still is soft, so that in the green tapes, there can be made cavities of desired shapes. Likewise, at desired spots, there can be silk-screened various electrically passive elements. The elastic layers are laminated together by means of pressure. In order to prevent the lamination pressure from collapsing the structure that contains various cavities, the pressurising must be carried out according to a so-called unaxial method. This means that the pressure is directed to the object only in the direction of the axis z of said object. Finally the resultant structure is burnt, in the case of LTCC at 850 degrees and in the case of HTCC at 1,600 degrees. In the elements to be produced, at the cavities there are made perforations through which the excess pressure created in the burning process can be let out.
In FIGS. 1a and 1b, there is illustrated a possible alternative for realising an inverted microstrip cable based on the HTCC or LTCC multilayer technique according to the description above. In a preferred embodiment, the structure according to FIG. 1a is achieved by joining together, during the production process but prior to the burning step of the structure, the exemplary elements 12 and 13 illustrated in the drawing. Both of said elements are made layer by layer of some suitable dielectric material in a fashion described above. In the element 13, there is machined a rectangular groove, on the bottom of which there is silk-screened a signal cable 10. The thickness 18 of the element 13, as shown in FIG. 1b, when measured at the bottom of the groove, is sufficient to prevent disturbing ground potential levels from coming close to the described inverted microstrip cable. In the example illustrated FIG. 1b, the angle between the side walls of the groove made in the element 13 and the groove bottom 16, 17 is 90 degrees, but in principle the angles can have some other size, too. On the surface of the element 12, there is silk-screened a ground cable 11, the width whereof corresponds to the width of the groove made in the element 13. The elements 12 and 13 are machined separately, and when they are connected, there is obtained a structure according to FIG. 1a, where there is created a gas-filled cable cavity 14.
In FIG. 1b, there is illustrated a cross-section made in the direction A-Axe2x80x2 of FIG. 1a. The attenuation and impedance of a transmission cable according to the invention are determined by the permitivity (∈r) of the employed elements 12 and 13, as well as the geometric shape of the groove. From the drawing it is seen that the electromagnetic field emitted from the signal cable 10, said field in the drawing being illustrated by the power lines 15, proceeds a long way inside the element 13. With RF frequencies, the permitivity of the element 13 is clearly higher than the permitivity of the gas mixture filling the cable cavity 14. This results in that the cable attenuation is strongly increased with RF frequencies. The final multilayer structure of the apparatus also includes other material layers than those illustrated in FIGS. 1a and 1b, in which layers there may be provided passive components, cavities for active components and other cable structures, too.
However, the use of electric circuits manufactured by the above described techniques becomes problematic, if very high frequencies must be used (RF applications). Signal attenuation in a cable structure realised with LTCC technique at the frequency of 20 GHz rises up to 0.2 dB/cm, and in a cable structure realised with HTCC technique up to 0.6 dB/cm. In such RF applications where low attenuation is required, for example in filters and oscillation sources having a high quality factor (Q value), the above described techniques are no longer feasible.
Another problem with regular microstrip cables or inverted microstrip cables is the impedance level of the transmission cables realised by means of structures. An uncontrolled fluctuation of the impedance level results in undesired reflections of the signal back in the direction where the signal came from, or in radiation in the cable surroundings. Impedance is affected by the geometric shape of the cable structure, as well as by the relative permittivity (∈r) of the surrounding material layers. In prior art structures, the above described two factors are the only free choices for adjusting the impedance.
With prior art LTCC and HTCC structures, another drawback is presented in the dispersion of the phase velocity with high frequencies. In a dispersed signal, the signal components contained therein at different frequencies have passed through the transmission cable at different velocities. This phenomenon distorts the received signal, and an excessive increase of the dispersion results in that the received signal becomes inapplicable.
From the U.S. Pat. No. 3,904,997 there is known an arrangement where the signal cable of an inverted microstrip resting on a substrate is encased in a shell-like structure made of metal. By means of this arrangement, both the attenuation of the transmission cable and the stray radiation scattered from the cable are attempted to be reduced. The metallic cable cavity must always be manufactured in advance, and its fastening in a reliable way to the rest of the multilayer structure causes problems. The fact that the thermal expansion coefficient of the metallic cable cavity is different from the basic substrate may cause the structure to break at the junction surface. Moreover, the structure includes a lot of manually performed work steps, wherefore it also is expensive in manufacturing costs.
From the U.S. Pat. No. 5,105,055 there is known an arrangement where in one flexible, cable-like structure there are integrated several cables. In said structure, the signal cable is attached to a dielectric substrate, and the ground cable is placed in a cavity-like structure made of another dielectric material. In principle, said cable is an entity made of several inverted microstrip cables. The materials of the cable structure are chosen among such materials that are elastic, and they can be processed with extrusion devices designed for processing plastics. Several variations of the cable structure are presented in the publication. According to said publication, the cable is meant to be used in connection with personal PC devices. Also in this case it is pointed out that owing to the target of usage, the materials chosen in the structure do not enable the use of RF frequencies.
The object of the invention is to reduce the described drawbacks connected to the prior art.
The transmission cable placed in a cavity according to the invention is characterised in that it comprises a support element with a surface essentially parallel to the first and second surfaces of the cavity, said support element being placed between said first and second surfaces, so that the signal cable is realised by means of an electroconductive material layer formed on the surface of said support element.
A number of preferred embodiments of the invention are set forth in the independent claims.
The basic principle of the invention is as follows: by applying multilayer technique, there is manufactured a modified, inverted microstrip cable, where the signal cable is attached, by means of a specially designed support element, on one surface of the cable cavity. Thus the effect of the material layers that encase the cable to the electromagnetic field surrounding said cable is remarkably reduced.
An advantage of the invention is that at RF frequencies the attenuation of a transmission cable according to the invention is clearly lower than with existing inverted microstrip cables, because the electromagnetic field emitted from the signal cable is mainly located in the gas-filled cable cavity, the permittivity (∈r) of said cable cavity with respect to the permittivity of the surrounding dielectric materials being low.
Another advantage of the invention is that the transmission cable can be fully integrated in a multilayer structure without any specific work steps carried out expressly for this purpose.
Yet another advantage of the invention is that thereby the impedance level of the transmission cable can be adjusted as desired in a simple fashion.